Retrospect and Prospect: reflections on forty years of study of aquatic oligochaetes

Abstract

Fourteen aquatic oligochaetes were described between 1773 and 1828. Dramatic increases in descriptions occurred in the decades beginning 1900, 1930 and 1960. These can be associated with specific authors, such as W. Michaelsen and S. Hrabě. In the period 1970–1990, some 250 marine species (excluding Enchytraeidae) were described, largely by C. Erséus. Some of the major scholars, such as W. Michaelsen and F. Vejdovský, I. I. MaleviČ and P. G. Svetlov, founded centers of oligochaete study that survive in Europe today. Others, such as those in the English speaking world, have had to develop in isolation. While numerical and phenetic methods had little influence on microdrile systematics, phylogenetic (parsimony) methods have, since the first publication by B. G. M. Jamieson. Most literature prior to 1970 was taxonomic. Other earlier work documented community structure in relation to pollution. The study of Rybinsk Reservoir (Russia) stimulated a large body of ecological and biological work by T. L. Poddubnaya. The St. Lawrence Great Lakes was also a focus for worm studies until recently. Early physiological work focused on respiration, with studies dating from the 1920s. This has been continued in relation to pollution ecology and energy flow. Physiological work was notably absent from our first proceedings volume and only poorly represented since then. Future prospects are difficult to identify. Developments in ultrastructure (including sperm and muscle cells), molecular biology and parsimony or cladistic phylogenetic methods will move from rather simple initial work that shows the possibilities, to a revisionary phase where the complexities begin to be revealed before it is possible to assess their value. This author has too superficial a level of understanding to predict the outcome, but suspects that agreement among the results of several methods is needed for them to be persuasive. I would hope that multivariate methods of data analysis in pollution studies would prove superior to the development of indices and over— reliance on toxicology. Toxicity studies are of use in comparing risks among products but not in predicting field effects. Benthic biology needs a method for identifying factors that are directly related to worm biology, such as food, rather than physical and chemical factors related to the water column. Surveys, relating communities to chemical and physiographic factors will remain unpredictive. It is easier to forsee some changes in taxonomy that are almost overdue. The haplotaxids should be broken up into several small families, Haplotaxis s.s. will be one of them, with a number of highly adapted and very similar species, many currently regarded as synonymous, redefined by careful anatomical study. The lumbriculids and branchiobdellidans may well be combined into a single taxon defined by their unique semiprosopore male ducts, with lumbriculids defined by their unique muscle structure and the branchiobdellids by many apomorphic characters related to ectocommensal life. The position of Hirudinea remains unresolved, but relationship to this taxon is not excluded. Detailed study of penes in lumbriculids is required. The naidids and tubificids seem to be a monophyly. Existing genera may be hard to analyze phylogenetically because definitions are based on atrial form, which is hard to express as characters. Chaetotaxy in Naididae will prove insufficient and many synonyms will be declared. Capilloventridae and Randiellidae require description of male ducts and gonads. They may provide evidence counter to the octogonadal theory of oligochaete descent, or of polyphyly in the group. Knowledge of phreodrilid diversity will increase. Work will increase in Asia and South America, but sub-Saharan Africa remains under explored. Fish diseases will create more interest in North American worms. New methods, including molecular studies, and renewed interest in ontogeny and detailed anatomical studies, may improve our ability to resolve clusters of taxa with few, if any, apomorphic character states that remain at the base of cladograms. An ability to include sound evidence from vestigial organs and logical arguments on a “weight of evidence”basis is needed as an adjunct. While molecular studies seem to hold much promise, early studies can prove over simplistic and can provide conflicting hypotheses. We need to complete the review of taxonomy according to parsimony before major terminology changes, such as abandonment of the term Oligochaeta, can gain acceptance.